Transistor having an oxide semiconductor film

ABSTRACT

To provide a thin film transistor which has high operation speed and in which a large amount of current can flow when the thin film transistor is on and off-state current at the time when the thin film transistor is off is extremely reduced. The thin film transistor is a vertical thin film transistor in which a channel formation region is formed using an oxide semiconductor film in which hydrogen or an OH group contained in the oxide semiconductor is removed so that hydrogen is contained in the oxide semiconductor at a concentration of lower than or equal to 5×10 19 /cm 3 , preferably lower than or equal to 5×10 18 /cm 3 , more preferably lower than or equal to 5×10 17 /cm 3 , and the carrier concentration is lower than or equal to 5×10 14 /cm 3 , preferably lower than or equal to 5×10 12 /cm 3 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 12/912,335, filed Oct. 26, 2010, now allowed, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2009-251060 on Oct. 30, 2009, both of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to field effect transistors, for example, thin film transistors in each of which an oxide semiconductor is used.

BACKGROUND ART

A technique for forming a thin film transistor (TFT) by using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. The thin film transistors have been used in display devices typified by a liquid crystal television. A silicon-based semiconductor material is known as a material for a semiconductor thin film that can be used in thin film transistors. Besides, an oxide semiconductor has attracted attention as another material.

As a material for the oxide semiconductor, zinc oxide or a substance containing zinc oxide is known. Thin film transistors each of which is formed using an amorphous oxide (an oxide semiconductor) having an electron carrier concentration of lower than 10¹⁸/cm³ are disclosed in Patent Documents 1 to 3.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No. 2006-165527

[Patent Document 2] Japanese Published Patent Application No. 2006-165528

[Patent Document 3] Japanese Published Patent Application No. 2006-165529

DISCLOSURE OF INVENTION

Deviation from the stoichiometric composition of an oxide semiconductor arises when the oxide semiconductor is formed as a thin film. For example, the electric conductivity of the oxide semiconductor is changed due to excess or deficiency of oxygen. In addition, hydrogen which is mixed into the oxide semiconductor during the thin film formation forms an oxygen (O)-hydrogen (H) bond and the OH bond serves as an electron donor, which is a factor for changing the electric conductivity. Furthermore, an OH group is polarized, and thus serves as a factor for changing characteristics of an active device such as a thin film transistor manufactured using the oxide semiconductor.

Even when the electron carrier concentration is lower than 10¹⁸/cm³, the oxide semiconductor is a substantially n-type oxide semiconductor, and the on/off ratio of the thin film transistors disclosed in the above patent documents is only 10³. The above low on/off ratio of the thin film transistors is caused by large off-state current.

In a display device, there is a problem in that unwanted charge build up in elements, electrodes, or wirings during manufacture or operation. In the case of, for example, a thin film transistor, such charge build-up might cause generation of a parasitic channel, which leads to flow of leakage current. Further, in the case of a bottom-gate transistor, charge might build up on a surface of or in a back channel portion in a semiconductor layer (i.e., a region of a semiconductor layer, which is sandwiched between a source electrode and a drain electrode which are formed over the semiconductor layer), so that a parasitic channel is likely to be generated and leakage current is likely to be generated, which cause variation in the threshold voltage.

The channel length along which carriers move may be reduced in order to increase the field-effect mobility of a thin film transistor. However, reducing the channel length causes an increase in off-state current of the thin film transistor.

In view of the above, it is an object of one embodiment of the present invention to provide a thin film transistor with high-speed operation in which a large amount of current can flow when the thin film transistor is on and off-state current is extremely reduced when the thin film transistor is off.

One embodiment of the present invention is a vertical thin film transistor in which a channel formation region is formed using an oxide semiconductor which has a larger energy gap than a silicon semiconductor and which is an intrinsic or substantially intrinsic semiconductor by removal of impurities which can be electron donors (donors) in the oxide semiconductor.

In other words, one embodiment of the present invention is a vertical thin film transistor in which a channel formation region is formed using an oxide semiconductor film in which hydrogen or an OH group contained in the oxide semiconductor is removed so that hydrogen is contained at a concentration of lower than or equal to 5×10¹⁹/cm³, preferably lower than or equal to 5×10¹⁸/cm³, more preferably lower than or equal to 5×10¹⁷/cm³, and the carrier concentration is lower than or equal to 5×10¹⁴/cm³, preferably lower than or equal to 5×10¹²/cm³.

The energy gap of the oxide semiconductor is greater than or equal to 2 eV, preferably greater than or equal to 2.5 eV, more preferably greater than or equal to 3 eV. Impurities such as hydrogen that forms a donor are reduced as much as possible so that the carrier concentration is lower than or equal to 1×10¹⁴/cm³, preferably lower than or equal to 1×10¹²/cm³.

Note that in one embodiment of the present invention, a plurality of gate electrodes in a thin film transistor face each other with a source electrode, an oxide semiconductor film, and a gate insulating film that covers a drain electrode interposed therebetween. In other words, the gate electrode faces side surfaces of the source electrode, the oxide semiconductor film, and the drain electrode with the gate insulating film therebetween. Therefore, the channel width is large.

According to one embodiment of the present invention, the use of an oxide semiconductor in which the hydrogen concentration is reduced and the purity is increased makes it possible to extremely reduce off-state current as well as to increase the field-effect mobility and on-state current of a thin film transistor.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1A is a top view of a thin film transistor and FIG. 1B is a cross-sectional view thereof;

FIG. 2 is a longitudinal cross-sectional view of an inverted staggered thin film transistor in which an oxide semiconductor is used;

FIGS. 3A and 3B are energy band diagrams (schematic diagrams) of layers of the thin film transistor in a cross-section taken along line A-A′ in FIG. 2;

FIG. 4A shows a state in which positive potential (+V_(G)) is applied to gates (GE1 and GE2) and FIG. 4B shows a state in which negative potential (−V_(G)) is applied to the gates (GE1 and GE2);

FIG. 5 shows a relation between the vacuum level and the work function of a metal (φ_(M)) and a relation between the vacuum level and the electron affinity (χ) of an oxide semiconductor;

FIG. 6A is a top view illustrating a thin film transistor and FIG. 6B is a cross-sectional view illustrating the same;

FIG. 7A is a top view illustrating a thin film transistor and FIG. 7B is cross-sectional view illustrating the same;

FIGS. 8A to 8E are cross-sectional views illustrating a method for manufacturing a thin film transistor;

FIGS. 9A and 9B are cross-sectional views illustrating a method for manufacturing the thin film transistor;

FIG. 10 is a top view illustrating a pixel of a display device;

FIG. 11 is a cross-sectional view illustrating a pixel of a display device;

FIGS. 12A1 and 12A2 are plan views of semiconductor devices and FIG. 12B is a cross-sectional view thereof;

FIG. 13 is a cross-sectional view of a semiconductor device;

FIG. 14A is a plan view of a semiconductor device and FIG. 14B is a cross-sectional view thereof;

FIGS. 15A to 15C illustrate electronic devices; and

FIGS. 16A to 16C illustrate electronic devices.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, Embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways without departing from the spirit and scope of the present invention. Therefore, the present invention is not construed as being limited to the description of Embodiments. In the structures to be given below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and explanation thereof will not be repeated.

Note that the size, the thickness of a layer, or a region of each structure illustrated in drawings in this specification is exaggerated for simplicity in some cases. Therefore, embodiments and examples of the present invention are not limited to such scales.

Further, the terms such as “first”, “second”, and “third” used in this specification are used just to avoid confusion of structural elements and do not mean limitation of the number of the structural elements. Therefore, for example, description can be made even when “first” is replaced with “second” or “third”, as appropriate.

Further, “voltage” refers to a difference between potential of two points, and “potential” refers to electrostatic energy (electrical potential energy) of a unit charge at a given point in an electrostatic field. Note that in general, an electric potential difference between potential of one point and reference potential (e.g., ground potential) is merely called potential or voltage, and potential and voltage are used as synonymous words in many cases. Thus, in this specification, potential may be rephrased as voltage and voltage may be rephrased as potential unless otherwise specified.

Embodiment 1

In this embodiment, a structure of a field effect transistor, for example, a thin film transistor will be described with reference to FIGS. 1A and 1B.

FIG. 1A is a top view of a thin film transistor 133. FIG. 1B is a cross-sectional view taken along chain line A-B in FIG. 1A.

As illustrated in FIG. 1B, a first electrode 105, an oxide semiconductor film 107, and a second electrode 109 are stacked over an insulating film 103 formed over a substrate 101. In addition, a gate insulating film 111 is provided so as to cover the first electrode 105, the oxide semiconductor film 107, and the second electrode 109. Third electrodes 113 and 115 are provided over the gate insulating film 111. An insulating film 117 that functions as an interlayer insulating film is provided over the gate insulating film 111 and the third electrodes 113 and 115. Openings are formed in the insulating film 117. A wiring 131 (see FIG. 1A) which is connected to the first electrode 105 through the opening is formed. A wiring 129 which is connected to the second electrode 109 through the opening, a wiring 125 which is connected to the third electrode 113 through the opening, and a wiring 127 which is connected to the third electrode 115 through the opening are formed. The first electrode 105 functions as one of a source electrode and a drain electrode of the thin film transistor. The second electrode 109 functions as the other of the source electrode and the drain electrode of the thin film transistor. The third electrodes 113 and 115 function as gate electrodes of the thin film transistor.

The thin film transistor according to this embodiment is a vertical thin film transistor, which has features that the third electrode 113 and the third electrode 115 each of which functions as a gate electrode are separated and that the third electrode 113 and the third electrode 115 face each other with the first electrode 105, the oxide semiconductor film 107, and the second electrode 109 interposed therebetween.

Note that a thin film transistor is an element that includes at least three terminals, including a gate, a drain, and a source, and includes a channel formation region between a drain region and a source region. Current can flow through the drain region, the channel region, and the source region. Here, since the source and drain of the thin film transistor might change depending on a structure, operating conditions, and the like of the thin film transistor, it is difficult to define which is a source or a drain. Therefore, a region that serves as a source or a drain is not referred to as a source or a drain in some cases. In such a case, for example, one of the source and the drain might be referred to as a first terminal and the other might be referred to as a second terminal. Alternatively, one of the source and the drain might be referred to as a first electrode and the other might be referred to as a second electrode. Further alternatively, one of the source and the drain might be referred to as a first region and the other might be referred to as a second region.

It is necessary that the substrate 101 have at least heat resistance high enough to withstand heat treatment performed later. As the substrate 101, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like can be used.

In the case where the temperature of the heat treatment performed later is high, a substrate whose strain point is 730° C. or higher is preferably used as a glass substrate. As a glass substrate, a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used, for example. Note that in general, in the case where a larger amount of barium oxide (BaO) than boron oxide (B₂O₃) is contained, a more practical heat-resistant glass substrate can be obtained. Thus, a glass substrate containing a larger amount of BaO than B₂O₃ is preferably used.

Note that a substrate formed of an insulator such as a ceramic substrate, a quartz substrate, or a sapphire substrate may be used instead of the above glass substrate. Besides, a crystallized glass substrate or the like can be used.

The insulating film 103 is formed using an oxide insulating film such as a silicon oxide film or a silicon oxynitride film, or a nitride insulating film such as a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, or an aluminum nitride oxide film. The insulating film 103 may have a stacked structure, for example, a stacked structure in which one or more of the above nitride insulating films and one or more of the above oxide insulating films are stacked in that order over the substrate 101.

The first electrode 105 and the second electrode 109 are formed using an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, tungsten, or yttrium; an alloy containing any of these elements as a component; an alloy containing these elements in combination; or the like. Alternatively, one or more materials selected from manganese, magnesium, zirconium, and beryllium can be used. In addition, the first electrode 105 can have a single-layer structure or a stacked structure having two or more layers. For example, a single-layer structure of an aluminum film containing silicon; a two-layer structure of an aluminum film and a titanium film stacked thereover; a two-layer structure of a tungsten film and a titanium film stacked thereover; a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in that order; and the like can be given. Alternatively, a film, an alloy film, or a nitride film which contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may be used.

Note that as the oxide semiconductor film 107, a thin film expressed by InMO₃(ZnO)_(m) (m>0, where m is not an integer) can be used. Here, M represents one or more metal elements selected from Ga, Fe, Ni, Mn, and Co. For example, M may be Ga, Ga and Ni, Ga and Fe, or the like. The oxide semiconductor film may contain a transition metal element or oxide of the transition metal element as an impurity element, in addition to the metal element contained as M. Among oxide semiconductor layers whose composition formulae are expressed by InMO₃(ZnO)_(m) (m>0 and m is not an integer), an oxide semiconductor which contains Ga as M is referred to as an In—Ga—Zn—O-based oxide semiconductor, and a thin film of the In—Ga—Zn—O-based oxide semiconductor is referred to as an In—Ga—Zn—O-based film.

As the oxide semiconductor film 107, any of the following oxide semiconductor films can be applied as well as the above In—Ga—Zn—O-based film: an In—Sn—Zn—O-based oxide semiconductor film; an In—Al—Zn—O-based oxide semiconductor film; a Sn—Ga—Zn—O-based oxide semiconductor film; an Al—Ga—Zn—O-based oxide semiconductor film; a Sn—Al—Zn—O-based oxide semiconductor film; an In—Zn—O-based oxide semiconductor film; a Sn—Zn—O-based oxide semiconductor film; an Al—Zn—O-based oxide semiconductor film; an In—O-based oxide semiconductor film; a Sn—O-based oxide semiconductor film; and a Zn—O-based oxide semiconductor film. Further, the oxide semiconductor film may further contain Si.

Hydrogen contained in the oxide semiconductor film 107 that is used in this embodiment is removed so that the concentration of hydrogen in the oxide semiconductor film 107 is lower than or equal to 5×10¹⁹/cm³, preferably lower than or equal to 5×10¹⁸/cm³, more preferably lower than or equal to 5×10¹⁷/cm³. In other words, the oxide semiconductor film is highly purified so that impurities that are not main components of the oxide semiconductor film are contained as little as possible. Further, the carrier concentration of the oxide semiconductor film 107 is lower than or equal to 5×10¹⁴/cm³, preferably lower than or equal to 1×10¹⁴/cm³, more preferably lower than or equal to 5×10¹²/cm³, further preferably lower than or equal to 1×10¹²/cm³. In other words, the carrier concentration of the oxide semiconductor film is as close to zero as possible. Further, the energy gap is greater than or equal to 2 eV, preferably greater than or equal to 2.5 eV, more preferably greater than or equal to 3 eV. Note that the hydrogen concentration in the oxide semiconductor film can be detected by secondary ion mass spectroscopy (SIMS). The carrier concentration can be measured by the Hall effect measurement.

The thickness of the oxide semiconductor film 107 is preferably greater than or equal to 30 nm and less than or equal to 3000 nm. The small thickness of the oxide semiconductor film 107 makes it possible to reduce the channel length of the thin film transistor, which leads to manufacture of the thin film transistor which has large on-state current and high electric-field mobility. In contrast, the large thickness of the oxide semiconductor film 107, typically a thickness of greater than or equal to 100 nm and less than or equal to 3000 nm, makes it possible to manufacture a semiconductor device for high power.

The gate insulating film 111 can be a single layer or a stack formed using a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, or an aluminum oxide film. A portion of the gate insulating film 111 which is in contact with the oxide semiconductor film 107 preferably contains oxygen, particularly preferably is formed of a silicon oxide film. The use of a silicon oxide film makes it possible to supply oxygen to the oxide semiconductor film 107; thus, favorable characteristics can be obtained.

The gate insulating film 111 is formed using a high-k material such as hafnium silicate (HfSiO_(x)), HfSiO_(x)N_(y) in which N is added, hafnium aluminate (HfAlO_(x)), hafnium oxide, or yttrium oxide, so that gate leakage current can be reduced. Further, a stacked structure in which a high-k material and one or more of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, and an aluminum oxide film are stacked can be used. The thickness of the gate insulating film 111 may be greater than or equal to 50 nm and less than or equal to 500 nm. The small thickness of the gate insulating film 111 makes it possible to manufacture a thin film transistor which has high field-effect mobility; thus, a driver circuit can be manufactured on the same substrate as the thin film transistor. In contrast, the large thickness of the gate insulating film 111 makes it possible to reduce leakage current.

The third electrodes 113 and 115 that function as gate electrodes can be formed using an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, or tungsten; an alloy containing any of these elements as a component; an alloy film containing these elements in combination; or the like. Alternatively, one or more materials selected from manganese, magnesium, zirconium, or beryllium may be used. In addition, the third electrodes 113 and 115 may have a single-layer structure or a stacked structure having two or more layers. For example, a single-layer structure of an aluminum film containing silicon; a two-layer structure of an aluminum film and a titanium film stacked thereover; a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in that order; and the like can be given. Alternatively, a film, an alloy film, or a nitride film which contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, or scandium may be used.

Next, operation of the thin film transistor in which the oxide semiconductor film 107 is included will be described with reference to energy band diagrams.

FIG. 2 is a longitudinal cross-sectional view of an inverted staggered thin film transistor in which the oxide semiconductor film that is described in this embodiment is used. An oxide semiconductor film (OS) and a source electrode (S) are stacked over a drain electrode (D). A gate insulating film (GI) is provided over the drain electrode, the oxide semiconductor film, and the source electrode, and gate electrodes (GE1 and GE2) that are separated are provided thereover.

FIGS. 3A and 3B are energy band diagrams (schematic diagrams) of the layers of the thin film transistor in cross section taken along line A-A′ in FIG. 2. FIG. 3A shows a case where voltage between the source and the drain is 0 V (V_(D)=0 V). FIG. 3B shows a case where in a state where positive voltage (V_(D)>0 V) is applied to the drain, voltage is not applied to a gate (V_(G)=0 V) (indicated by dashed lines) and positive voltage (V_(G)>0 V) is applied to the gate (indicated by solid lines). When no voltage is applied to the gate, carriers (electrons) are not injected from the electrode to the oxide semiconductor side due to the high potential barrier, resulting in an off state in which no current flows. In contrast, when positive voltage is applied to the gate, the potential barrier is lowered, resulting in an on state where current flows.

FIGS. 4A and 4B are energy band diagrams (schematic diagrams) of the layers of the thin film transistor in cross section taken along line B-B′ in FIG. 2. FIG. 4A shows a state where positive potential (+V_(G)) is applied to the gates (GE1 and GE2), that is, a case where the thin film transistor is in an on state (a conducting state) where carriers (electrons) flow between the source and the drain. FIG. 4B shows a state where negative potential (−V_(G)) is applied to the gates (GE1 and GE2), that is, a case where the thin film transistor is in an off state (a non-conducting state, where minority carriers do not flow).

FIG. 5 shows a relation between the vacuum level and the work function of a metal (φ_(M)) and a relation between the vacuum level and the electron affinity (χ) of the oxide semiconductor film.

At room temperature, a free electron of metal is in a degenerate state, and the Fermi level is located in the conduction band. On the other hand, a conventional oxide semiconductor film is generally of n-type, and the Fermi level (E_(F)) in that case is located closer to the conduction band and is away from the intrinsic Fermi level (Ei) that is located in the middle of the band gap. Note that it is known that some hydrogen contained in the oxide semiconductor film form a donor and is one factor that causes an oxide semiconductor film to be an n-type oxide semiconductor film.

In contrast, the oxide semiconductor film according to this embodiment is an oxide semiconductor film that is made to be an intrinsic (i-type) semiconductor film or made to be close to an intrinsic semiconductor film by being highly purified by removal of hydrogen that is an n-type impurity so that impurities that are not main components of the oxide semiconductor film are contained as little as possible. In other words, the oxide semiconductor film according to this embodiment has a feature that it is made to be an i-type (intrinsic) semiconductor or made to be close thereto by being highly purified by removal of as many impurities such as hydrogen, water, hydroxyl groups, or hydride as possible, instead of addition of impurities. As a result, the Fermi level (E_(F)) can be at the same level as the intrinsic Fermi level (Ei).

It is said that the electron affinity (χ) of an oxide semiconductor film is 4.3 eV in the case where the band gap (Eg) thereof is 3.15 eV. The work function of titanium (Ti) used for forming the source and drain electrodes is substantially equal to the electron affinity (χ) of the oxide semiconductor film. In the case where titanium is used for the source and drain electrodes, the Schottky electron barrier is not formed at an interface between the metal and the oxide semiconductor film.

In other words, an energy band diagram (a schematic diagram) like FIG. 3A is obtained in the case where the work function of the metal (φ_(m)) and the electron affinity (χ) of the oxide semiconductor film are substantially equal and the metal and the oxide semiconductor film are in contact with each other.

In FIG. 3B, a black circle (•) represents an electron. When positive potential is applied to the drain, the electrons cross a barrier (h) to be injected into the oxide semiconductor film and flow toward the drain. In that case, the height of the barrier (h) changes depending on the gate voltage and the drain voltage. In the case where positive drain voltage is applied, the height of the barrier (h) is smaller than the height of the barrier (h) in FIG. 3A of the case where no voltage is applied; that is, the height of the barrier (h) is smaller than half of the band gap (Eg).

In this case, as shown in FIG. 4A, the electron moves along the lowest part of the oxide semiconductor film, which is energetically stable, at an interface between the gate insulating film and the highly-purified oxide semiconductor film.

In FIG. 4B, when negative potential (reverse bias voltage) is applied to the gates (GE1 and GE2), the number of holes that are minority carriers is substantially zero; thus, the current value becomes a value that is extremely close to zero.

For example, even when the thin film transistor has a channel width W of 1×10⁴ μm and a channel length of 3 μm, an off-state current of 10⁻¹³ A or less, which is extremely low, and a subthreshold value (S value) of 0.1 V/dec. (the thickness of the gate insulating film: 100 nm) can be obtained.

As described above, the oxide semiconductor film is highly purified so that impurities that are not main components of the oxide semiconductor film, typically hydrogen, water, hydroxyl groups, hydride, or the like, are contained as little as possible, whereby favorable operation of the thin film transistor can be obtained. In particular, off-state current can be reduced.

In the meantime, a horizontal thin film transistor in which a channel is formed in substantially parallel to a substrate needs a source and a drain as well as the channel; thus, the area occupied by the thin film transistor in the substrate is increased, which hinders miniaturization. However, in a vertical thin film transistor, a source, a channel, and a drain are stacked, whereby the area occupied by the thin film transistor in a substrate surface can be reduced. As a result, the thin film transistor can be miniaturized.

The channel length of the vertical thin film transistor can be controlled by the thickness of the oxide semiconductor film; thus, a small thickness of the oxide semiconductor film 107 makes it possible to manufacture a thin film transistor with a short channel length. A short channel length makes it possible to reduce the series resistance of the source, the channel, and the drain; thus, the on-state current and field-effect mobility of the thin film transistor can be increased. Further, a thin film transistor in which an oxide semiconductor film in which the hydrogen concentration is reduced and the purity is increased is included has extremely small off-state current and is in an insulating state where substantially no current flows when the thin film transistor is off. Thus, even when the thickness of the oxide semiconductor film is reduced to reduce the channel length of the vertical thin film transistor, a thin film transistor in which substantially no off-state current flows in a non-conducting state can be obtained.

As described above, the oxide semiconductor film in which the hydrogen concentration is reduced and the purity is increased is used, whereby a thin film transistor which is suitable for increase in definition and has high operation speed and in which a large amount of current can flow when the thin film transistor is on and substantially no current flows when the thin film transistor is off can be manufactured.

Embodiment 2

In this embodiment, a structure of a field effect transistor, for example, a thin film transistor that is different from that of the thin film transistor described in Embodiment 1 will be described with reference to FIGS. 6A and 6B.

FIG. 6A is a top view of thin film transistors 141 and 143. FIG. 6B is a cross-sectional view taken along chain line A-B in FIG. 6A.

As illustrated in FIG. 6B, the first electrodes 105 and 106, the oxide semiconductor film 107, and the second electrode 109 are stacked over the insulating film 103 formed over the substrate 101. In addition, the gate insulating film 111 is provided so as to cover the first electrodes 105 and 106, the oxide semiconductor film 107, and the second electrode 109. The third electrodes 113 and 115 are provided over the gate insulating film 111. The insulating film 117 that functions as an interlayer insulating film is provided over the gate insulating film 111 and the third electrodes 113 and 115. Openings are formed in the insulating film 117. The wiring 131 (see FIG. 6A) which is connected to the first electrode 105 through one of the openings, a wiring 132 (see FIG. 6A) which is connected to the first electrode 106 through one of the openings, a wiring 129 which is connected to the second electrode 109 through one of the openings, the wiring 125 which is connected to the third electrode 113 through one of the openings, and the wiring 127 which is connected to the third electrode 115 through one of the openings are formed.

The first electrode 105 functions as one of a source electrode and a drain electrode of the thin film transistor 141. The first electrode 106 functions as one of a source electrode and a drain electrode of the thin film transistor 143. The second electrode 109 functions as the other of the source electrode and the drain electrode of each of the thin film transistors 141 and 143. The third electrode 113 functions as a gate electrode of the thin film transistor 141. The third electrode 115 functions as a gate electrode of the thin film transistor 143.

This embodiment has a feature that the first electrode 105 and the first electrode 106 are separated from each other. Further, this embodiment has a feature that the thin film transistor 141 and the thin film transistor 143 are connected in series through the second electrode 109 and the wiring 129. In that case, the first electrode 105 functions as one of a source electrode and a drain electrode (e.g., the source) of the thin film transistor 141. The second electrode 109 functions as the other of the source electrode and the drain electrode (e.g., the drain) of the thin film transistor 141. The third electrode 113 functions as a gate electrode of the thin film transistor 141. Further, the second electrode 109 functions as one of a source electrode and a drain electrode (e.g., the source) of the thin film transistor 143. The first electrode 106 functions as the other of the source electrode and the drain electrode (e.g., the drain) of the thin film transistor 143. The third electrode 115 functions as a gate electrode of the thin film transistor 143.

In other words, the thin film transistor 141 and the thin film transistor 143 are connected in series through the second electrode 109. In that case, the wiring 129 does not have to be provided.

Alternatively, the thin film transistor 141 and the thin film transistor 143 may be connected in parallel. In that case, a conductive layer for connecting the first electrode 105 to the second electrode 106 is provided.

As in the thin film transistor described in Embodiment 1, in the thin film transistors 141 and 143 in this embodiment, an oxide semiconductor film in which the hydrogen concentration is reduced and the purity is increased is used. Thus, favorable operation of the thin film transistor can be obtained. In particular, off-state current can be reduced. As a result, a thin film transistor can be manufactured which is suitable for increase in definition and has high operation speed and in which a large amount of current can flow when the thin film transistor is on and substantially no current flows when the thin film transistor is off state.

Embodiment 3

In this embodiment, a field effect transistor, for example, a thin film transistor that is one embodiment of the present invention and is different from the thin film transistors described in the above embodiments will be described.

FIGS. 7A and 7B illustrate an example of a thin film transistor according to this embodiment.

The thin film transistor illustrated in FIGS. 7A and 7B has a stacked structure that is similar to that of the thin film transistor described in Embodiment 1. In other words, first electrodes 105A to 105D, the oxide semiconductor film 107, and the second electrode 109 are stacked over the insulating film 103 formed over the substrate 101. In addition, the gate insulating film 111 is provided so as to cover the first electrodes 105A to 105D, the oxide semiconductor film 107, and the second electrode 109. Third electrodes 113A to 113D are provided over the gate insulating film 111. The insulating film 117 that functions as an interlayer insulating film is provided over the gate insulating film 111 and the third electrodes 113A to 113D. Openings are formed in the insulating film 117. Wirings 125A to 125D which are connected to the first electrodes 105A to 105D respectively through the corresponding one of the openings are formed. The wiring 129 which is connected to the second electrode 109 through the corresponding one of the opening is formed. The first electrodes 105A to 105D each functions as one of a source electrode and a drain electrode of the thin film transistor. The second electrode 109 functions as the other of the source electrode and the drain electrode of the thin film transistor. The third electrodes 113A to 113D each function as a gate electrode of the thin film transistor.

The thin film transistor in this embodiment is a vertical thin film transistor. The third electrodes 113A to 113D that function as gate electrodes are separated from one another. The third electrode 113A faces the third electrode 113C with the first electrodes 105A and 105C, the oxide semiconductor film 107, and the second electrode 109 interposed therebetween. The third electrode 113B faces the third electrode 113D with the first electrodes 105B and 105D, the oxide semiconductor film 107, and the second electrode 109 interposed therebetween.

The thin film transistor 133 illustrated in FIGS. 7A and 7B includes a plurality of thin film transistors. Specifically, the thin film transistor 133 includes four thin film transistors 141A to 141D.

In the thin film transistor 141A, a gate is formed of the third electrode 113A, one of a source and a drain is formed of the first electrode 105A, and the other of the source and the drain is formed of the second electrode 109. In the thin film transistor 141B, a gate is formed of the third electrode 113B, one of a source and a drain is formed of the first electrode 105B, and the other of the source and the drain is formed of the second electrode 109. In the thin film transistor 141C, a gate is formed of the third electrode 113C, one of a source and a drain is formed of the first electrode 105C, and the other of the source and the drain is formed of the second electrode 109. In the thin film transistor 141D, a gate is formed of the third electrode 113D, one of a source and a drain is formed of the first electrode 105D, and the other of the source and the drain is formed of the second electrode 109.

Note that materials and formation methods of the layers described in this embodiment are the same as those in Embodiment 1; therefore, the description of the materials and the formation methods is not given here.

As in Embodiment 1, in the thin film transistor 133 in this embodiment, an oxide semiconductor film in which the hydrogen concentration is reduced and the purity is increased is used. Thus, favorable operation of the thin film transistor can be obtained. In particular, off-state current can be reduced. As a result, a thin film transistor can be manufactured which is suitable for increase in definition and has high operation speed and in which a large amount of current can flow when the thin film transistor is in a conducting state and substantially no current flows when the thin film transistor is in a non-conducting state.

Embodiment 4

In this embodiment, a manufacturing process of the thin film transistor in FIGS. 1A and 1B will be described with reference to FIGS. 8A to 8E.

As illustrated in FIG. 8A, the insulating film 103 is formed over the substrate 101, and the first electrode 105 is formed over the insulating film 103. The first electrode 105 functions as one of the source electrode and the drain electrode of the thin film transistor.

The insulating film 103 can be formed by a sputtering method, a CVD method, a coating method, or the like.

Note that in the case where the insulating film 103 is formed by a sputtering method, the insulating film 103 is preferably formed while hydrogen, water, hydroxyl groups, hydride, or the like remaining in a treatment chamber is removed. This is for preventing hydrogen, water, hydroxyl groups, hydride, or the like from being contained in the insulating film 103. An entrapment vacuum pump is preferably used in order to remove hydrogen, water, hydroxyl groups, hydride, or the like remaining in the treatment chamber. As the entrapment vacuum pump, for example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. An evacuation unit may be a turbo pump provided with a cold trap. Impurities, particularly, hydrogen, water, hydroxyl groups, hydride, or the like are removed in the treatment chamber which is evacuated with a cryopump; thus, when the insulating film 103 is formed in the treatment chamber, the concentration of impurities contained in the insulating film 103 can be reduced.

As a sputtering gas used for forming the insulating film 103, a high-purity gas is preferably used in which impurities such as hydrogen, water, hydroxyl groups, or hydride are reduced to such a level that the impurity concentration is represented by the unit “ppm” or “ppb”.

Examples of a sputtering method include an RF sputtering method in which a high-frequency power source is used for a sputtering power source, a DC sputtering method in which a DC power source is used, and a pulsed DC sputtering method in which a bias is applied in a pulsed manner. An RF sputtering method is mainly used in the case where an insulating film is formed, and a DC sputtering method is mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in which a plurality of targets of different materials can be set. With the multi-source sputtering apparatus, films of different materials can be formed to be stacked in the same chamber, or a film of plural kinds of materials can be formed by electric discharge at the same time in the same chamber.

In addition, there are a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering method and a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge.

Furthermore, as a sputtering method, a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during deposition to form a thin compound film thereof, and a bias sputtering method in which voltage is also applied to a substrate during deposition can be used.

As the sputtering process in this specification, the above-described sputtering apparatus and sputtering method can be employed as appropriate.

In this embodiment, a silicon oxide film is formed as the insulating film 103 over the substrate 101 in such a manner that the substrate 101 is introduced into the treatment chamber; a sputtering gas which contains high-purity oxygen and from which hydrogen, water, hydroxyl groups, hydride, or the like is removed is introduced into the treatment chamber; and a silicon target is used. When the insulating film 103 is formed, the substrate 101 may be heated.

For example, the silicon oxide film is formed by an RF sputtering method under conditions that quartz (preferably, synthesized quartz) is used; the substrate temperature is 108° C., the distance between the substrate and the target (the T-S distance) is 60 mm; the pressure is 0.4 Pa; the high-frequency power is 1.5 kW; and the atmosphere contains oxygen and argon (the flow ratio of oxygen to argon is 1:1 (each flow rate is 25 sccm)). The thickness of the silicon oxide film may be 100 nm. Note that a silicon target can be used instead of quartz (preferably, synthesized quartz). Note that as the sputtering gas, oxygen, or a mixed gas of oxygen and argon is used.

In the case where the insulating film 103 is formed to have a stacked structure, for example, a silicon nitride film is formed using a silicon target and a sputtering gas which contains high-purity nitrogen and from which hydrogen, water, hydroxyl groups, hydride, or the like is removed, between the silicon oxide film and the substrate. Also in this case, it is preferable that a silicon nitride film be formed while hydrogen, water, hydroxyl groups, hydride, or the like remaining in the treatment chamber is removed in a manner similar to that of the formation of the silicon oxide film. Note that in this step, the substrate 101 may be heated.

In the case where the silicon nitride film and the silicon oxide film are stacked to form the insulating film 103, the silicon nitride film and the silicon oxide film can be formed in the same treatment chamber using the same silicon target. First, a sputtering gas containing nitrogen is introduced and the silicon nitride layer is formed using a silicon target provided in the treatment chamber is used, and then the sputtering gas is switched to a sputtering gas containing nitrogen and a silicon nitride film is formed using the same silicon target. The silicon nitride film and the silicon oxide film can be formed successively without being exposed to air; thus, impurities such as hydrogen, water, hydroxyl groups, or hydride can be prevented from being adsorbing onto a surface of the silicon nitride layer.

The first electrode 105 can be formed in such a manner that a conductive film is formed over the insulating film 103 by a sputtering method, a CVD method, or a vacuum evaporation method, a resist mask is formed over the conductive film in a photolithography step, and the conductive film is etched using the resist mask. Alternatively, the first electrode 105 is formed by a printing method or an ink-jet method without using a photolithography step, which leads to reduction in the number of steps. Note that end portions of the first electrode 105 are preferably in a tapered shape, in which case the coverage with a gate insulating film that is formed later is improved. The angle formed between the end portion of the first electrode 105 and the insulating film 103 is set to greater than or equal to 30° and less than or equal to 60°, preferably greater than or equal to 40° and less than or equal to 50°, whereby the coverage with the gate insulating film that is formed later can be improved.

In this embodiment, as the conductive film that serves as the first electrode 105, a titanium film is formed to a thickness of 50 nm by a sputtering method, an aluminum film is formed to a thickness of 100 nm, and a titanium film is formed to a thickness of 50 nm. Next, etching is performed using the resist mask formed in the photolithography step, whereby the island-shaped first electrode 105 is formed.

Next, as illustrated in FIG. 8B, the oxide semiconductor film 107 and the second electrode 109 are formed over the first electrode 105. The oxide semiconductor film 107 functions as a channel formation region of the thin film transistor, and the second electrode 109 functions as the other of the source electrode and the drain electrode of the thin film transistor.

Here, a method for forming the oxide semiconductor film 107 and the second electrode 109 will be described.

An oxide semiconductor film is formed by a sputtering method over the substrate 101 and the first electrode 105. Next, a conductive film is formed over the oxide semiconductor film.

In order to prevent hydrogen from being contained in the oxide semiconductor film 107 as much as possible, it is preferable that as pretreatment, the substrate 101 over which the first electrode 105 is formed is preheated in a preheating chamber of a sputtering apparatus so that impurities such as hydrogen, water, hydroxyl groups, or hydride attached to the substrate 101 can be eliminated and removed. As an evacuation unit provided for the preheating chamber, a cryopump is preferably used. Note that this preheating step can be omitted. This preheating step may be performed on the substrate 101 over which the gate insulating film 111 has not been formed or may be performed on the substrate 101 over which the third electrodes 113 and 115 have not been formed.

Note that before the oxide semiconductor film is formed by a sputtering method, reverse sputtering in which plasma is generated by introduction of an argon gas is preferably performed to clean the surface of the first electrode 105, in which case the resistance at an interface between the first electrode 105 and the oxide semiconductor film can be reduced. The reverse sputtering refers to a method in which, without application of voltage to a target side, a high-frequency power source is used for application of voltage to a substrate side in an argon atmosphere to generate plasma in the vicinity of the substrate to modify a surface. Note that a nitrogen atmosphere, a helium atmosphere, or the like may be used instead of an argon atmosphere.

In this embodiment, the oxide semiconductor film is formed by a sputtering method with the use of an In—Ga—Zn—O-based oxide semiconductor target for film formation. The oxide semiconductor film can be formed by a sputtering method in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas (typically argon) and oxygen. When a sputtering method is employed, a target containing SiO₂ at greater than or equal to 2 wt % and less than or equal to 10 wt % may be used.

As a sputtering gas used for forming the oxide semiconductor film, a high-purity gas is preferably used in which impurities such as hydrogen, water, hydroxyl groups, or hydride are reduced to such a level that the impurity concentration is represented by the unit “ppm” or “ppb”.

As a target used for forming the oxide semiconductor film by a sputtering method, a target of metal oxide containing zinc oxide as a main component can be used. As another example of the metal oxide target, an oxide semiconductor target for film formation containing In, Ga, and Zn (at a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 (mol %), In:Ga:Zn=1:1:0.5 (atom %)). Alternatively, as an oxide semiconductor target for film formation containing In, Ga, and Zn, a target having such a composition ratio that In:Ga:Zn=1:1:1 (atom %) or In:Ga:Zn=1:1:2 (atom %) can be used. The filling rate of the oxide semiconductor target for film formation is greater than or equal to 90% and less than or equal to 100%, preferably, greater than or equal to 95% and less than or equal to 99.9%. An oxide semiconductor film formed using the oxide semiconductor target for film formation with high filling rate is dense.

The oxide semiconductor film is formed over the insulating film 103 and the second electrode 109 in such a manner that a sputtering gas from which hydrogen, water, hydroxyl groups, hydride, or the like is removed is introduced into the treatment chamber and a metal oxide is used as a target while the substrate is held in the treatment chamber kept at reduced pressure and moisture remaining in the treatment chamber is removed. An entrapment vacuum pump is preferably used in order to remove hydrogen, water, hydroxyl groups, hydride, or the like remaining in the treatment chamber. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. An evacuation unit may be a turbo pump provided with a cold trap. In the treatment chamber evacuated with a cryopump, for example, hydrogen, water, hydroxyl groups, hydride (more preferably a compound containing a carbon atom), or the like is eliminated; thus, the concentration of impurities contained in the oxide semiconductor film can be reduced. The oxide semiconductor film may be formed while the substrate is heated.

In this embodiment, as an example of the film formation condition of the oxide semiconductor film, the following conditions are applied: the substrate temperature is room temperature, the distance between the substrate and the target is 110 mm; the pressure is 0.4 Pa; the direct current (DC) power is 0.5 kW; and the atmosphere contains oxygen and argon (oxygen flow rate of 15 sccm:argon flow rate of 30 sccm). Note that a pulsed direct-current (DC) power source is preferably used, in which case powder substances (also referred to as particles or dust) generated in film formation can be reduced and the thickness can be uniform. The thickness of the oxide semiconductor film is preferably greater than or equal to 30 nm and less than or equal to 3000 nm. Note that the appropriate thickness of the oxide semiconductor film differs depending on the material to be used; therefore, the thickness may be determined as appropriate in accordance with the material.

Note that the sputtering method and sputtering apparatus that are used for forming the insulating film 103 can be used as appropriate as a sputtering method and a sputtering apparatus for forming the oxide semiconductor film.

The conductive film that serves as the second electrode 109 can be formed using the material and method that are used for forming the first electrode 105, as appropriate. Here, as the conductive film that serves as the second electrode 109, a 50-nm-thick titanium film, a 100-nm-thick aluminum film, and a 50-nm-thick titanium film are stacked in that order.

Next, a resist mask is formed over the conductive film in a photolithography step, the conductive film that serves as the second electrode 109 and the oxide semiconductor film that serves as the oxide semiconductor film 107 are etched using the resist mask, whereby the island-shaped second electrode 109 and island-shaped oxide semiconductor film 107 are formed. Note that formation of a resist mask by an ink-jet method instead of the resist mask formed in the photolithography step leads to reduction in the number of steps. The angle formed between the first electrode 105 and the end portions of the second electrode 109 and the oxide semiconductor film 107 is set to greater than or equal to 30° and less than or equal to 60°, preferably greater than or equal to 40° and less than or equal to 50° by this etching, whereby the coverage with the gate insulating film that is formed later can be improved.

Note that the etching of the conductive film and the oxide semiconductor film here may be performed using either dry etching or wet etching, or using both dry etching and wet etching. In order to form the oxide semiconductor film 107 and the second electrode 109 with desired shapes, etching conditions (etchant, etching time, temperature, and the like) are adjusted as appropriate in accordance with a material.

When the etching rate of each of the oxide semiconductor film and the conductive film that serves as the second electrode 109 is different from that of the first electrode 105, conditions under which the etching rate of the first electrode 105 is low and the etching rate of each of the oxide semiconductor film and the conductive film that serves as the second electrode 109 is high are selected. Alternatively, conditions under which the etching rate of the oxide semiconductor film is low and the etching rate of the conductive film that serves as the second electrode 109 is high are selected to etch the conductive film that serves as the second electrode 109, and then conditions under which the etching rate of the first electrode 105 is low and the etching rate of the oxide semiconductor film is high are selected.

As an etchant used for wet etching the oxide semiconductor film, a mixed solution of phosphoric acid, acetic acid, and nitric acid, an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2), or the like can be used. In addition, ITO07N (produced by KANTO CHEMICAL CO., INC.) may also be used.

The etchant after the wet etching is removed together with the etched materials by cleaning. The waste liquid containing the etchant and the material etched off may be purified and the material may be reused. When a material such as indium contained in the oxide semiconductor film is collected from the waste liquid after the etching and reused, the resources can be efficiently used and the cost can be reduced.

As an etching gas used for dry etching of the oxide semiconductor film, a gas containing chlorine (a chlorine-based gas such as chlorine (Cl₂), boron trichloride (BCl₃), silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferably used.

Alternatively, a gas containing fluorine (fluorine-based gas such as carbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogen trifluoride (NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr); oxygen (O₂); any of these gases to which a rare gas such as helium (He) or argon (Ar) is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching) method or an ICP (inductively coupled plasma) etching method can be used. In order to etch the films into desired shapes, the etching condition (the amount of electric power applied to a coil-shaped electrode, the amount of electric power applied to an electrode on a substrate side, the temperature of the electrode on the substrate side, or the like) is adjusted as appropriate.

In this embodiment, the conductive film that serve as the second electrode 109 is etched using an ammonia hydrogen peroxide mixture (a mixture of ammonia, water, and hydrogen peroxide water) as an etchant, and then the oxide semiconductor film is etched using a solution in which phosphoric acid, acetic acid, and nitric acid are mixed, whereby the oxide semiconductor film 107 is formed.

Next, in this embodiment, first heat treatment is performed. The first heat treatment is performed at a temperature higher than or equal to 400° C. and lower than or equal to 750° C., preferably, higher than or equal to 400° C. and lower than the strain point of the substrate. Here, the substrate is introduced into an electric furnace that is a kind of heat treatment apparatus and heat treatment is performed on the oxide semiconductor film in an atmosphere of an inert gas such as nitrogen or a rare gas at 450° C. for one hour, and then the oxide semiconductor film is not exposed to air. Accordingly, hydrogen, water, hydroxyl groups, or hydride can be prevented from entering the oxide semiconductor film, the hydrogen concentration is reduced, and the purity is increased. As a result, an i-type oxide semiconductor film or a substantially i-type oxide semiconductor film can be obtained. In other words, at least one of dehydration and dehydrogenation of the oxide semiconductor film 107 can be performed by this first heat treatment.

Note that it is preferable that in the first heat treatment, hydrogen, water, hydroxyl groups, hydride, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. Alternatively, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is preferably higher than or equal to 6N (99.9999%), more preferably higher than or equal to 7N (99.99999%) (that is, the concentration of the impurities is 1 ppm or lower, preferably 0.1 ppm or lower).

The oxide semiconductor film in some cases might be crystallized to become a microcrystalline film or a polycrystalline film depending on the conditions of the first heat treatment or the material of the oxide semiconductor film. For example, the oxide semiconductor layer might be crystallized to become a microcrystalline oxide semiconductor film having a degree of crystallization of 90% or more, or 80% or more. Further, depending on the conditions of the first heat treatment and the material of the oxide semiconductor film, the oxide semiconductor film might become amorphous oxide semiconductor film containing no crystalline component. The oxide semiconductor film might become an oxide semiconductor film in which a microcrystalline portion (with a grain diameter greater than or equal to 1 nm and less than or equal to 20 nm, typically greater than or equal to 2 nm and less than or equal to 4 nm) is mixed into the amorphous oxide semiconductor film.

Alternatively, the first heat treatment of the oxide semiconductor film may be performed on the oxide semiconductor film before being processed into the island-shaped oxide semiconductor film. In that case, the substrate is taken out of the heating apparatus after the first heat treatment, and then a photolithography step is performed.

The heat treatment which has an effect of dehydrating or dehydrogenating the oxide semiconductor layer may be performed at any of the following timings: after the oxide semiconductor film is formed; after the conductive film that serves as the second electrode is stacked over the oxide semiconductor film; after the gate insulating film is formed over the first electrode, the oxide semiconductor film, and the second electrode; and after the gate electrode is formed.

Next, as illustrated in FIG. 8C, the gate insulating film 111 is formed over the first electrode 105, the oxide semiconductor film 107, and the second electrode 109.

The oxide semiconductor film that is made to be intrinsic oxide semiconductor film or a substantially intrinsic oxide semiconductor film by removal of impurities (the oxide semiconductor film in which the hydrogen concentration is reduced and the purity is increased) is extremely sensitive to an interface state and interface charge; thus, the interface between the oxide semiconductor film and the gate insulating film 111 is important. Therefore, the gate insulating film 111 that is in contact with the highly-purified oxide semiconductor film needs high quality.

For example, a high-density plasma CVD using microwaves (2.45 GHz) is preferably used, in which case an insulating film which is dense, has high withstand voltage, and has high quality can be formed. The oxide semiconductor film in which the hydrogen concentration is reduced and the purity is increased is in close contact with the high-quality gate insulating film, whereby the interface state density can be reduced to obtain favorable interface characteristics.

Needless to say, another film formation method such as a sputtering method or a plasma CVD method can be employed as long as the method enables formation of a good-quality insulating film as a gate insulating film. Alternatively, an insulating film whose film quality and characteristic of an interface between the insulating film and an oxide semiconductor film are improved by heat treatment which is performed after formation of the gate insulating film may be formed as a gate insulating film. In any case, any insulating film may be used as long as the insulating film has characteristics of enabling reduction in interface state density of an interface between the insulating film and the oxide semiconductor film and formation of a favorable interface as well as having favorable film quality as a gate insulating film.

Further, when an oxide semiconductor film containing impurities is subjected to a gate bias-temperature stress test (BT test) for 12 hours under conditions that the temperature is 85° C. and the voltage applied to the gate is 2×10⁶ V/cm, a bond between the impurity and a main component of the oxide semiconductor film is cleaved by a high electric field (B: bias) and a high temperature (T: temperature), and a generated dangling bond induces drift of threshold voltage (V_(th))

In contrast, one embodiment of the present invention makes it possible to obtain a thin film transistor which is stable to a BT test by removing impurities in an oxide semiconductor film, especially hydrogen or water as much as possible to obtain a favorable characteristic of an interface between the oxide semiconductor film and a gate insulating film as described above

When the gate insulating film 111 is formed by a sputtering method, the concentration of hydrogen in the gate insulating film 111 can be reduced. In the case where a silicon oxide film is formed by a sputtering method, a silicon target or a quartz target is used as a target and oxygen or a mixed gas of oxygen and argon is used as a sputtering gas.

Impurities such as hydrogen, water, hydroxyl groups, or hydride (also referred to as a hydrogen compound) existing in the oxide semiconductor film or at the interface between the oxide semiconductor film and the insulating film that is provided in contact with the oxide semiconductor film may be removed in such a manner that a halogen element (e.g., fluorine or chlorine) is contained in the insulating film provided in contact with the oxide semiconductor film, or a halogen element is contained in an oxide semiconductor film by plasma treatment in a gas atmosphere containing a halogen element with the oxide semiconductor film exposed. In the case where the insulating film contains a halogen element, the concentration of the halogen element in the insulating film may be approximately 5×10¹⁸ atoms/cm³ to 1×10²⁰ atoms/cm³.

As described above, in the case where a halogen element is contained in the oxide semiconductor film or at the interface between the oxide semiconductor film and the insulating film which is in contact with the oxide semiconductor film and the insulating film which is provided in contact with the oxide semiconductor film is an oxide insulating film, the oxide insulating film on the side where the oxide semiconductor film is not in contact with the oxide insulating film is preferably covered with a nitrogen insulating film. In other words, a silicon nitride film or the like may be provided on and in contact with the oxide insulating film which is on and in contact with the oxide semiconductor film. Such a structure makes it possible to prevent impurities such as hydrogen, water, hydroxyl groups, hydride from entering the oxide insulating film.

The gate insulating film 111 can have a structure in which a silicon oxide film and a silicon nitride film are stacked in that order over the first electrode 105, the oxide semiconductor film 107, and the second electrode 109. For example, a gate insulating film with a thickness of 100 nm may be formed in such a manner that a silicon oxide film (SiO_(x) (x>0)) is formed to a thickness of greater than or equal to 5 nm and less than or equal to 300 nm as the first gate insulating film, and a silicon nitride film (SiN_(y) (y>0)) is formed to a thickness of greater than or equal to 50 nm and less than or equal to 200 nm as the second gate insulating film over the first gate insulating film by a sputtering method. In this embodiment, the silicon oxide film is formed to a thickness of 100 nm is formed by an RF sputtering method under conditions that the pressure is 0.4 Pa; the high-frequency power is 1.5 kW; and the atmosphere contains oxygen and argon (the flow ratio of oxygen to argon is 1:1 (each flow rate is 25 sccm)).

Next, second heat treatment may be performed in an inert gas atmosphere or an oxygen gas atmosphere (preferably, at a temperature higher than or equal to 200° C. and lower than or equal to 400° C., for example, a temperature higher than or equal to 250° C. and lower than or equal to 350° C.). Note that the second heat treatment may be performed after formation of any of the third electrodes 113 and 115, the insulating film 117, or the wirings 125, 127, and 129, which is performed later. This heat treatment allows hydrogen or water contained in the oxide semiconductor film to be diffused into the oxide semiconductor film.

Next, the third electrodes 113 and 115 that function as a gate electrode are formed over the gate insulating film 111.

The third electrodes 113 and 115 can be formed in such a manner that a conductive film that serves as the third electrodes 113 and 115 is formed over the gate insulating film 111 by a CVD method or a vacuum evaporation method, a resist mask is formed over the conductive film in a photolithography step, and the conductive film is etched using the resist mask.

In this embodiment, after a titanium film is formed to a thickness of 150 nm by a sputtering method, and then the titanium film is etched using a resist mask formed in a photolithography step, whereby the third electrodes 113 and 115 are formed.

Through the above-described steps, the thin film transistor 133 in which the oxide semiconductor film 107 in which the hydrogen concentration is reduced and the purity is increased is used can be manufactured.

Next, as illustrated in FIG. 8D, the insulating film 117 is formed over the gate insulating film 111 and the third electrodes 113 and 115, and then contact holes 119, 121, and 123 are formed.

The insulating film 117 is formed using an oxide insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, or an aluminum oxynitride film; or a nitride insulating film such as a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, or an aluminum nitride oxide film. Alternatively, the insulating film 117 can be a stack of an oxide insulating film and a nitride insulating film.

The insulating film 117 is formed by a sputtering method, a CVD method, or the like. Note that in the case where the insulating film 117 is formed by a sputtering method, an insulating film may be formed in such a manner that the substrate 101 is heated to a temperature of 100° C. to 400° C., a sputtering gas from which hydrogen, water, hydroxyl groups, hydride, or the like is removed and which contains high-purity nitrogen is introduced, and a silicon target is used. Also in this case, an insulating film is preferably formed while hydrogen, water, hydroxyl groups, hydride, or the like remaining in the treatment chamber is removed.

Note that after the insulating film 117 is formed, heat treatment may be further performed in air at a temperature higher than or equal to 100° C. and lower than or equal to 200° C. for greater than or equal to 1 hour and less than or equal to 30 hours. This heat treatment enables a normally-off thin film transistor to be obtained. Thus, the reliability of a display device or a semiconductor device can be increased.

A resist mask is formed in a photolithography step, and parts of the gate insulating film 111 and the insulating film 117 are removed by selective etching, whereby the contact holes 119, 121, and 123 that reach the first electrode 105, the second electrode 109, and the third electrodes 113 and 115 are formed.

Next, a conductive film is formed over the gate insulating film 111 and in the contact holes 119, 121, and 123, and then the conductive film is etched using a resist mask formed in a photolithography step, whereby the wirings 125, 127, 129, and 131 are formed. Note that the resist mask may be formed by an ink-jet method. Formation of the resist mask by an inkjet method needs no photomask, which results in a reduction in manufacturing costs.

The wirings 125, 127, 129, and 131 can be formed in a manner similar to that of the first electrode 105.

Note that a planarization insulating film for planarization may be provided between the third electrodes 113 and 115 and the wirings 125, 127, 129, and 131. An organic material having heat resistance, such as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy can be used as typical examples of the planarization insulating film. Other than the above organic materials, it is also possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. Note that the planarization insulating film may be formed by stacking a plurality of insulating films formed from these materials.

Note that the siloxane-based resin corresponds to a resin including a Si—O—Si bond formed using a siloxane-based material as a starting material. The siloxane-based resin may include an organic group (e.g., an alkyl group or an aryl group) or a fluoro group as a substituent. Moreover, the organic group may include a fluoro group.

There is no particular limitation on the method for forming the planarization insulating film. The planarization insulating film can be formed, depending on the material, by a method such as a sputtering method, an SOG method, a spin coating method, a dipping method, a spray coating method, or a droplet discharge method (e.g., an ink-jet method, screen printing, or offset printing), or with a tool such as a doctor knife, a roll coater, a curtain coater, or a knife coater.

As described above, the concentration of hydrogen in the oxide semiconductor film can be reduced and the oxide semiconductor film is highly purified. Thus, stabilization of the oxide semiconductor film can be achieved. In addition, heat treatment at a temperature lower than or equal to the glass transition temperature makes it possible to form an oxide semiconductor film with a wide band gap in which the number of minority carriers is extremely small. Thus, thin film transistors can be manufactured using a large substrate, which leads to mass production. Further, the use of the oxide semiconductor film in which the hydrogen concentration is reduced and the purity is increased makes it possible to manufacture a thin film transistor which is suitable for increase in definition and has high operation speed and in which a large amount of current can flow when the thin film transistor is on and substantially no current flows when the thin film transistor is off.

This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

Embodiment 5

In this embodiment, a field effect transistor, for example, a thin film transistor in which an oxide semiconductor film that is different from the oxide semiconductor film described in Embodiment 4 is included will be described with reference to FIGS. 8A and 8B and FIGS. 9A and 9B.

In a manner similar to that in Embodiment 4, as illustrated in FIG. 8A, the insulating film 103 and the first electrode 105 are formed over the substrate 101. Next, as illustrated in FIG. 8B, the oxide semiconductor film 107 and the second electrode 109 are formed over the first electrode 105.

Next, first heat treatment is performed. The first heat treatment in this embodiment is different from the first heat treatment in the above embodiment. The heat treatment makes it possible to form the crystal grains in the surface of an oxide semiconductor film 151 illustrated in FIG. 9B. In this embodiment, the first heat treatment is performed with an apparatus for heating an object to be processed by at least one of thermal conduction and thermal radiation from a heater such as a resistance heater. Here, the temperature of the heat treatment is higher than or equal to 500° C. and lower than or equal to 700° C., preferably higher than or equal to 650° C. and lower than or equal to 700° C. Note that, although there is no requirement for the upper limit of the heat treatment temperature from the essential part of the invention, the upper limit of the heat treatment temperature needs to be within the allowable temperature limit of the substrate 101. In addition, the length of time of the heat treatment is preferably greater than or equal to 1 minute and less than or equal to 10 minutes. When RTA treatment is employed for the first heat treatment, the heat treatment can be performed in a short time; thus, adverse effects of heat on the substrate 101 can be reduced. In other words, the upper limit of the heat treatment temperature can be raised in this case as compared with the case where heat treatment is performed for a long time. In addition, the crystal grains having predetermined structures can be selectively formed in the vicinity of the surface of the oxide semiconductor film.

As examples of the heat treatment apparatus that can be used in this embodiment, rapid thermal anneal (RTA) apparatuses such as a gas rapid thermal anneal (GRTA) apparatus and a lamp rapid thermal anneal (LRTA) apparatus, and the like are given. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with an object to be processed by heat treatment, such as nitrogen or a rare gas such as argon is used.

For example, as the first heat treatment, GRTA may be performed in which the substrate is moved into an atmosphere of an inert gas such as nitrogen or a rare gas which has been heated to a temperature as high as 650° C. to 700° C.; heated for several minutes; and moved out of the inert gas which has been heated to a high temperature. GRTA enables high-temperature heat treatment to be performed in a short time.

Note that in the first heat treatment, it is preferable that hydrogen, water, hydroxyl groups, hydride, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. Alternatively, the purity of nitrogen or a rare gas such as helium, neon, or argon that is introduced into the heat treatment apparatus is preferably 6N (99.9999%) or higher, more preferably 7N (99.99999%) or higher (that is, the impurity concentration is 1 ppm or lower, preferably 0.1 ppm or lower).

Note that the above heat treatment may be performed at any timing as long as it is performed after the oxide semiconductor film 107 is formed; however, in order to promote dehydration or dehydrogenation, the heat treatment is preferably performed before other components are formed on a surface of the oxide semiconductor film 107. In addition, the heat treatment may be performed plural times instead of once.

FIG. 9B is an enlarged view of a dashed line portion 153 in FIG. 9A.

The oxide semiconductor film 151 includes an amorphous region 155 that mainly contains an amorphous oxide semiconductor and crystal grains 157 that are formed in the surface of the oxide semiconductor film 151. Further, the crystal grains 157 are formed in a region that extends from the surface of the oxide semiconductor film 151 to a depth of 20 nm or less (i.e., the vicinity of the surface). Note that the location where the crystal grains 157 are formed is not limited to the above in the case where the thickness of the oxide semiconductor film 151 is large. For example, in the case where the oxide semiconductor film 151 has a thickness of 200 nm or more, the “vicinity of a surface (surface vicinity)” means a region that has a thickness (depth) from the surface of the oxide semiconductor film 151, which is 10% or less of the thickness of the oxide semiconductor film.

Here, the amorphous region 155 mainly contains an amorphous oxide semiconductor film. Note that the word “mainly” means, for example, a state where one occupies 50% or more of a region. In this case, it means a state where the amorphous oxide semiconductor film occupies 50% or more at volume % (or weight %) of the amorphous region 155. In other words, the amorphous region in some cases includes crystals of an oxide semiconductor film other than an amorphous oxide semiconductor film, and the percentage of the content thereof is preferably less than 50% at volume % (or weight %). However, the percentage of the content is not limited to the above.

In the case where an In—Ga—Zn—O-based oxide semiconductor is used as a material for the oxide semiconductor film, the composition of the above amorphous region 155 is preferably set so that the Zn content (atomic %) is less than the In or Ga content (atomic %) for the reason that such composition makes it easy for the crystal grains 157 which have predetermined composition to be formed.

After that, a gate insulating film and a third electrode that functions as a gate electrode are formed in a manner similar to that of Embodiment 4.

The vicinity of the surface of the oxide semiconductor film 151, which is in contact with the gate insulating film, serves as a channel. The crystal grains are included in the region that serves as a channel, whereby the resistance between a source, the channel, and a drain is reduced and carrier mobility is increased. Thus, the field-effect mobility of the thin film transistor in which the oxide semiconductor film 151 is included is increased, which leads to favorable electric characteristics of the thin film transistor.

Further, the crystal grains 157 are more stable than the amorphous region 155; thus, when the crystal grains 157 are included in the vicinity of the surface of the oxide semiconductor film 151, entry of impurities (e.g., hydrogen, water, hydroxyl groups, or hydride) into the amorphous region 155 can be reduced. Thus, the reliability of the oxide semiconductor film 151 can be improved.

Through the above-described steps, the concentration of hydrogen in the oxide semiconductor film can be reduced and the oxide semiconductor film is highly purified. Thus, stabilization of the oxide semiconductor film can be achieved. In addition, heat treatment at a temperature lower than or equal to the glass transition temperature makes it possible to form an oxide semiconductor film with a wide band gap in which the number of minority carriers is extremely small. Thus, thin film transistors can be manufactured using a large substrate, which leads to the enhancement of mass production. Further, the use of the oxide semiconductor film in which the hydrogen concentration is reduced and the purity is increased makes it possible to manufacture a thin film transistor which is suitable for increase in definition and has high operation speed and in which a large amount of current can flow when the thin film transistor is on and substantially no current flows when the thin film transistor is off.

This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

Embodiment 6

In this embodiment, a manufacturing process of the thin film transistor illustrated in FIGS. 1A and 1B will be described with reference to FIGS. 8A to 8E.

In a manner similar to that of Embodiment 4, as illustrated in FIG. 8A, the first electrode 105 is formed over the substrate 101.

Next, as illustrated in FIG. 8B, the oxide semiconductor film 107 and the second electrode 109 are formed over the first electrode 105.

Note that before the oxide semiconductor film is formed by a sputtering method, reverse sputtering in which plasma is generated by introduction of an argon gas is preferably performed so that dust or an oxide film which is attached to a surface of the first electrode 105 is removed, in which case the resistance at an interface between the first electrode 105 and the oxide semiconductor film can be reduced. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, or the like may be used.

The oxide semiconductor film is formed over the substrate 101 and the first electrode 105 by a sputtering method. Then, a conductive film is formed over the oxide semiconductor film.

In this embodiment, the oxide semiconductor film is formed by a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target for formation. In this embodiment, the substrate is held in a treatment chamber which is maintained in a reduced pressure state, and the substrate is heated to room temperature or a temperature lower than 400° C. Then, the oxide semiconductor film is formed over the insulating film 103 and the first electrode 105 in such a manner that a sputtering gas from which hydrogen, water, hydroxyl groups, hydride, or the like is removed is introduced while hydrogen, water, hydroxyl groups, hydride, or the like remaining in the treatment chamber is removed. An entrapment vacuum pump is preferably used for removing hydrogen, water, hydroxyl groups, hydride, or the like remaining in the treatment chamber. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. An evacuation unit may be a turbo pump provided with a cold trap. In the treatment chamber evacuated with a cryopump, for example, hydrogen, water, hydroxyl groups, hydride (more preferably a compound containing a carbon atom), or the like is eliminated; thus, the concentration of impurities contained in the oxide semiconductor film formed in the treatment chamber can be reduced. Further, sputtering formation is performed while hydrogen, water, hydroxyl groups, hydride, or the like remaining in the treatment chamber is removed with a cryopump, whereby an oxide semiconductor film in which impurities such as hydrogen atoms and water are reduced can be formed even at a substrate temperature of room temperature to a temperature lower than 400° C.

In this embodiment, film formation conditions that the distance between the substrate and the target is 100 mm, the pressure is 0.6 Pa, the direct-current (DC) power is 0.5 kW, and the atmosphere is an oxygen atmosphere (the proportion of oxygen flow is 100%) are employed. Note that a pulsed direct-current (DC) power source is preferably used, in which case powdery substances (also referred to as particles or dust) which are generated at the time of film formation can be reduced and the film thickness can be uniform. The oxide semiconductor film preferably has a thickness of greater than or equal to 30 nm and less than or equal to 3000 nm. Note that the appropriate thickness of the oxide semiconductor film differs depending on the material to be used; therefore, the thickness may be determined as appropriate in accordance with the material.

Note that the sputtering method and sputtering apparatus that are used for forming the insulating film 103 can be used as appropriate as a sputtering method and a sputtering apparatus for forming the oxide semiconductor film.

Next, a conductive film that serves as the second electrode 109 is formed using the material and method that are used for forming the first electrode 105.

Next, in a manner similar to that of Embodiment 4, the conductive film that serves as the second electrode 109 and the oxide semiconductor film that serves as the oxide semiconductor film 107 are etched, whereby the island-shaped second electrode 109 and the island-shaped oxide semiconductor film 107 are formed. The etching conditions (such as an etchant, etching time, and temperature) are adjusted as appropriate in accordance with the material in order to form the oxide semiconductor film 107 and the second electrode 109 with desired shapes.

Next, as illustrated in FIG. 8C, in a manner similar to that of Embodiment 4, the gate insulating film 111 is formed over the first electrode 105, the oxide semiconductor film 107, and the second electrode 109. As the gate insulating film 111, a gate insulating film that has a favorable characteristic of an interface between the gate insulating film 111 and the oxide semiconductor film 107 is preferable. The gate insulating film 111 is preferably formed by high-density plasma CVD method using microwaves (2.45 GHz), in which case the gate insulating film 111 can be dense and can have high withstand voltage and high quality. Another method such as a sputtering method or a plasma CVD method can be employed as long as the method enables a good-quality insulating film to be formed as the gate insulating film.

Note that before the gate insulating film 111 is formed, reverse sputtering is preferably performed so that resist residues and the like attached to at least a surface of the oxide semiconductor film 107 are removed.

Further, before the gate insulating film 111 is formed, hydrogen, water, hydroxyl groups, hydride, or the like attached to an exposed surface of the oxide semiconductor film may be removed by plasma treatment using a gas such as N₂O, N₂, or Ar. Alternatively, plasma treatment may be performed using a mixed gas of oxygen and argon. In the case where plasma treatment is performed, the gate insulating film 111 which is to be in contact with part of the oxide semiconductor film is preferably formed without being exposed to air.

Further, it is preferable that the substrate 101 over which components up to and including the first electrode 105 to the second electrode 109 are formed be preheated in a preheating chamber in a sputtering apparatus as pretreatment to remove hydrogen, water, hydroxyl groups, hydride, or the like attached to the substrate 101 so that hydrogen, water, hydroxyl groups, hydride, or the like is contained as little as possible in the gate insulating film 111. Alternatively, it is preferable that the substrate 101 be preheated in a preheating chamber in a sputtering apparatus to eliminate and remove impurities such as hydrogen, water, hydroxyl groups, hydride, or the like attached to the substrate 101 after the gate insulating film 111 is formed. Note that the temperature of the preheating is higher than or equal to 100° C. and lower than or equal to 400° C., preferably higher than or equal to 150° C. and lower than or equal to 300° C. A cryopump is preferable as an evacuation unit provided in the preheating chamber. Note that this preheating treatment can be omitted.

The gate insulating film 111 can have a structure in which a silicon oxide film and a silicon nitride film are stacked in that order over the first electrode 105, the oxide semiconductor film 107, and the second electrode 109. For example, a silicon oxide film (SiO_(x) (x>0)) is formed to a thickness of greater than or equal to 5 nm and less than or equal to 300 nm as a first gate insulating film by a sputtering method and a silicon nitride film (SiN_(y) (y>0)) is formed to a thickness of greater than or equal to 50 nm and less than or equal to 200 nm as a second gate insulating film over the first gate insulating film, whereby the gate insulating film 111 is formed.

Next, as illustrated in FIG. 8C, in a manner similar to that of Embodiment 4, the third electrodes 113 and 115 that function as gate electrodes are formed over the gate insulating film 111.

Through the above-described steps, the thin film transistor 133 in which the oxide semiconductor film 107 in which the hydrogen concentration is reduced is included can be manufactured.

Hydrogen, water, hydroxyl groups, hydride, or the like remaining in a reaction atmosphere is removed in forming the oxide semiconductor film as described above, whereby the concentration of hydrogen in the oxide semiconductor film can be reduced. Thus, stabilization of the oxide semiconductor film can be achieved.

Next, as illustrated in FIG. 8D, in a manner similar to that of Embodiment 4, the contact holes 119, 121, and 123 are formed after the insulating film 117 is formed over the gate insulating film 111 and the third electrodes 113 and 115.

Next, as illustrated in FIG. 8E, in a manner similar to that of Embodiment 4, the wirings 125, 127, and 129 are formed.

Note that in a manner similar to that of Embodiment 4, after the formation of the insulating film 117, heat treatment may be further performed at a temperature higher than or equal to 100° C. and lower than or equal to 200° C. in air for greater than or equal to 1 hour and less than or equal to 30 hours. This heat treatment enables a normally-off thin film transistor to be obtained. Thus, the reliability of a display device or a semiconductor device can be improved.

Note that a planarization insulating film for planarization may be provided between the third electrodes 113 and 115 and the wirings 125, 127, and 129.

Hydrogen, water, hydroxyl groups, hydride, or the like remaining in a reaction atmosphere is removed in forming the oxide semiconductor film as described above, whereby the concentration of hydrogen in the oxide semiconductor film can be reduced and the purity of the oxide semiconductor film can be increased. Thus, stabilization of the oxide semiconductor film can be achieved. In addition, heat treatment at a temperature lower than or equal to the glass transition temperature makes it possible to form an oxide semiconductor film with a wide band gap in which the number of minority carriers is extremely small. Thus, thin film transistors can be manufactured using a large substrate, which leads to the enhancement of mass production. Further, the use of the oxide semiconductor film in which the hydrogen concentration is reduced and the purity is increased makes it possible to manufacture a thin film transistor which is suitable for increase in definition and has high operation speed and in which a large amount of current can flow when the thin film transistor is on and substantially no current flows when the thin film transistor is off.

This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

Embodiment 7

In this embodiment, an element substrate and a pixel structure in a display device in which the thin film transistor described in any of the above embodiments is used will be described with reference to FIG. 10 and FIG. 11.

FIG. 10 is a top view of a pixel 160 in a display portion in a display device. FIG. 11 is a cross-sectional view taken along dashed lines A-B and C-D in FIG. 10.

Although the thin film transistor 133 described in Embodiment 1 is used as a pixel thin film transistor for controlling potential of a pixel electrode for description in this embodiment, any of the thin film transistors described in the other embodiments can be used as appropriate. The first electrode 105 that functions as one of a source electrode and a drain electrode of the thin film transistor 133 is connected to a pixel electrode 167. The second electrode 109 that functions as the other of the source electrode and the drain electrode of the thin film transistor 133 is connected to a signal line 161 through a conductive film 165. In addition, a capacitor wiring 163 is formed of the layer that is used for forming the first electrode 105. The conductive film 165 and the pixel electrode 167 are formed over a planarization insulating film 171 for planarization.

Note that in the thin film transistors described in Embodiments 1 to 6, the oxide semiconductor film in which the hydrogen concentration is reduced and the purity is increased is used; thus, off-state current of the thin film transistors is low. Thus, a capacitor for holding signal voltage applied to the pixel electrode does not have to be additionally provided. In other words, the capacitor wiring 163 does not need to be provided; thus, the aperture ratio of the pixel can be increased.

The planarization insulating film 171 can be formed using the material of the planarization insulating film, which is described in Embodiment 4, as appropriate.

The pixel electrode 167 is formed using a conductive film that is favorable to each display device.

The element substrate described in this embodiment can be used as appropriate in other display devices such as a liquid crystal display device, a light-emitting display device, and an electrophoretic display device. In addition, the structure of the pixel is not limited to the structure illustrated in FIG. 10 and FIG. 11, and a thin film transistor, a diode, and a capacitor can be provided as appropriate.

This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

Embodiment 8

In this embodiment, the case where a thin film transistor is manufactured and the thin film transistor is used in a pixel portion and a peripheral circuit portion (e.g., a driver circuit) so that a semiconductor device having a display function (a display device) is manufactured will be described. Part of or all the peripheral circuit portion is formed over a substrate where the pixel portion is formed, whereby a system-on-panel can be realized.

The display device includes a display element. As the display element, a liquid crystal element (also referred to as a liquid crystal display element), a light-emitting element (also referred to as a light-emitting display element), or the like can be used. The light-emitting element includes, in its category, an element whose luminance is controlled by current or voltage, and specifically includes, in its category, an inorganic electroluminescent (EL) element, an organic EL element, and the like. Further, a display medium whose contrast is changed by an electric effect, such as electronic ink, may be used.

In addition, the display device includes a panel in which the display element is sealed, and a module in which an IC including a controller or the like is mounted on the panel. Furthermore, an element substrate included in a display device is provided with a unit for supplying current to the display element in each of pixel portions. Specifically, the element substrate may be in a state after only a pixel electrode of the display element is formed, or in a state after a conductive layer to be a pixel electrode is formed and before the conductive layer is etched.

Hereinafter, in this embodiment, an example of a liquid crystal display device will be described. FIGS. 12A1 and 12A2 are plan views and FIG. 12B is a cross-sectional view of a panel in which thin film transistors 4010 and 4011 and a liquid crystal element 4013 that are formed over a first substrate 4001 are sealed by a second substrate 4006 and a sealant 4005. Here, FIGS. 12A1 and 12A2 are each a plan view and FIG. 12B is a cross-sectional view taken along line M-N in FIGS. 12A1 and 12A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 and a scan line driver circuit 4004 that are provided over the first substrate 4001. The second substrate 4006 is provided over the pixel portion 4002 and the scan line driver circuit 4004. In other words, the pixel portion 4002 and the scan line driver circuit 4004 are sealed together with a liquid crystal layer 4008, by the first substrate 4001, the sealant 4005, and the second substrate 4006. Further, a signal line driver circuit 4003 that is formed using a single crystal semiconductor or a polycrystalline semiconductor over a substrate separately prepared is mounted in a region that is different from the region surrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method of a driver circuit that is separately formed, and a COG method, a wire bonding method, a TAB method, or the like can be used as appropriate. FIG. 12A1 illustrates an example of mounting the signal line driver circuit 4003 by a COG method, and FIG. 12A2 illustrates an example of mounting the signal line driver circuit 4003 by a TAB method.

Further, the pixel portion 4002 and the scan line driver circuit 4004 provided over the first substrate 4001 each include a plurality of thin film transistors. FIG. 12B illustrates the thin film transistor 4010 included in the pixel portion 4002 and the thin film transistor 4011 included in the scan line driver circuit 4004. An insulating film 4020 is provided over the thin film transistors 4010 and 4011.

As the thin film transistors 4010 and 4011, any of the thin film transistors that are described in the above embodiments, or the like can be employed.

A pixel electrode 4030 included in the liquid crystal element 4013 is electrically connected to the thin film transistor 4010. A counter electrode 4031 of the liquid crystal element 4013 is provided for the second substrate 4006. The liquid crystal element 4013 is constituted by the pixel electrode 4030, the counter electrode 4031, and the liquid crystal layer 4008. Note that the pixel electrode 4030 and the counter electrode 4031 are provided with an insulating film 4032 and an insulating film 4033, respectively, each of which functions as an alignment film. The liquid crystal layer 4008 is sandwiched between the pixel electrode 4030 and the counter electrode 4031 with the insulating films 4032 and 4033 interposed therebetween.

Note that the substrate 101 that is described in Embodiment 1 can be used as the first substrate 4001 and the second substrate 4006 as appropriate. Alternatively, metal (typically stainless steel), ceramic, plastic, or the like can be used. As plastic, a fiberglass-reinforced plastics (FRP) substrate, a polyvinyl fluoride (PVF) film, a polyester film, an acrylic resin film, or the like can be used. Further alternatively, a sheet in which aluminum foil is sandwiched by PVF films or polyester films can be used.

A columnar spacer 4035 is provided in order to control the distance (cell gap) between the pixel electrode 4030 and the counter electrode 4031. The columnar spacer 4035 can be obtained by selective etching of an insulating film. Note that a spherical spacer may be used instead of the columnar spacer. The counter electrode 4031 is electrically connected to a common potential line formed over the same substrate as the thin film transistor 4010. For example, the counter electrode 4031 can be electrically connected to the common potential line through conductive particles provided between the pair of substrates. Note that the conductive particles are preferably contained in the sealant 4005.

Alternatively, a liquid crystal showing a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of the liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase is only generated within a narrow range of temperatures, a liquid crystal composition containing a chiral agent at 5 wt % or more is preferably used. Thus, the temperature range can be improved. The liquid crystal composition which includes a liquid crystal showing a blue phase and a chiral agent has a small response time of 10 μs to 100 μs, has optical isotropy, which makes the alignment process unneeded, and has a small viewing angle dependence.

Although an example of a transmissive liquid crystal display device is described in this embodiment, the present invention is not limited thereto, and a reflective liquid crystal display device or a semi-transmissive liquid crystal display device may be formed.

As the example of the liquid crystal display device described in this embodiment, a polarizing plate is provided on the outer surface of the substrate (on the viewer side) and a coloring layer and an electrode used for a display element are provided on the inner surface of the substrate; however, the polarizing plate may be provided on the inner surface of the substrate. The stacked structure of the polarizing plate and the coloring layer is not limited to this embodiment and may be set as appropriate depending on materials of the polarizing plate and the coloring layer or conditions of manufacturing process. Further, a black mask (a black matrix) may be provided as a light-shielding film.

Although the thin film transistor obtained in any of the above embodiments is covered with the insulating film 4020 in order to reduce surface unevenness caused by the thin film transistor in this embodiment, the invention disclosed is not limited to this structure.

The insulating film 4020 can be formed using the material of the planarization insulating film, which is described in Embodiment 4, as appropriate.

The pixel electrode 4030 and the counter electrode 4031 can be formed using a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added.

A conductive composition containing a conductive high molecule (also referred to as a conductive polymer) may be used for the pixel electrode 4030 and the counter electrode 4031. The pixel electrode formed of the conductive composition has preferably a sheet resistance of less than or equal to 1.0×10⁴ Ω/square and a transmittance of greater than or equal to 70% at a wavelength of 550 nm. Furthermore, the resistivity of the conductive high molecule contained in the conductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called π-electron conjugated conductive polymer can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, a copolymer of two or more kinds of them, and the like can be given.

A variety of signals are supplied from an FPC 4018 to the signal line driver circuit 4003, the scan line driver circuit 4004, the pixel portion 4002, or the like.

In addition, a connection terminal electrode 4015 is formed from the same conductive film as the pixel electrode 4030 included in the liquid crystal element 4013, and a terminal electrode 4016 is formed from the same conductive film as a source or drain electrode of the thin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to a terminal included in the FPC 4018 through an anisotropic conductive film 4019.

Note that FIGS. 12A1, 12A2 and 12B illustrate the example in which the signal line driver circuit 4003 is formed separately and then mounted on the first substrate 4001; however, this embodiment is not limited to this structure. The scan line driver circuit may be separately formed and then mounted, or only part of the signal line driver circuit or part of the scan line driver circuit may be separately formed and then mounted.

This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

Embodiment 9

In this embodiment, active matrix electronic paper that is an example of a semiconductor device will be described with reference to FIG. 13. A thin film transistor 650 used in a semiconductor device can be manufactured in a manner similar to that of any of the thin film transistors described in the above embodiments.

The electronic paper illustrated in FIG. 13 is an example of a display device in which a twist ball display method is employed. The twist ball display method refers to a method in which spherical particles each colored in black and white are arranged between a first electrode and a second electrode, and a potential difference is generated between the first electrode and the second electrode, whereby orientation of twist balls is controlled so that display is performed.

The thin film transistor 650 provided over the substrate 600 is a thin film transistor according to one embodiment of the invention disclosed and has a structure in which an oxide semiconductor film is sandwiched between a source or drain electrode which is located above the oxide semiconductor film and a source or drain electrode which is located below the oxide semiconductor film. Note that the source or drain electrode is electrically connected to a first electrode 660 through a contact hole formed in an insulating film. A substrate 602 is provided with a second electrode 670. Twist balls 680 each having a black region 680 a and a white region 680 b are provided between the first electrode 660 and the second electrode 670. A space around the twist balls 680 is filled with a filler 682 such as a resin (see FIG. 13). In FIG. 13, the first electrode 660 corresponds to a pixel electrode, and the second electrode 670 corresponds to a common electrode. The second electrode 670 is electrically connected to a common potential line provided over the substrate where the thin film transistor 650 is formed.

Instead of the twist ball, an electrophoretic display element can be used. In that case, for example, a microcapsule having a diameter of approximately 10 μm to 200 μm in which transparent liquid, positively-charged white microparticles, and negatively-charged black microparticles are encapsulated, is used. When an electric field is applied by the first electrode and the second electrode, the white microparticles and the black microparticles move to opposite sides from each other, whereby a white or black image is displayed. The electrophoretic display element has higher reflectance than a liquid crystal display element, and thus, an auxiliary light is unnecessary and a display portion can be recognized in a place where brightness is not sufficient. In addition, there is an advantage that an image that has been displayed once can be maintained even when power is not supplied to the display portion.

As described above, the use of the invention disclosed makes it possible to manufacture high-performance electronic paper. This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

Embodiment 10

In this embodiment, an example of a light-emitting display device will be described as a semiconductor device. As a display element included in a display device, a light-emitting element utilizing electroluminescence will be described here. Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter is referred to as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and current flows. Then, the carriers (electrons and holes) recombine, so that light is emitted. Owing to this mechanism, the light-emitting element is called a current-excitation light-emitting element.

The inorganic EL elements are classified, according to their element structures, into a dispersion-type inorganic EL element and a thin-film inorganic EL element. A dispersion-type inorganic EL element has a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination type light emission that utilizes a donor level and an acceptor level. A thin-film inorganic EL element has a structure where a light-emitting layer is sandwiched between dielectric layers, which are further sandwiched between electrodes, and its light emission mechanism is localized type light emission that utilizes inner-shell electron transition of metal ions. Note that an example in which an organic EL element is used as a light-emitting element is described here.

Next, the appearance and a cross section of a light-emitting display panel (also referred to as a light-emitting panel), which corresponds to one embodiment of the semiconductor device, are described with reference to FIGS. 14A and 14B. FIG. 14A is a plan view and FIG. 14B is a cross-sectional view of a panel in which thin film transistors 4509 and 4510 and a light-emitting element 4511 that are formed over a first substrate 4501 are sealed by a second substrate 4506 and a sealant 4505. Here, FIG. 14A is a plan view and FIG. 14B is a cross-sectional view taken along line H-I in FIG. 14A.

A sealant 4505 is provided to surround a pixel portion 4502, signal line driver circuits 4503 a and 4503 b, and scan line driver circuits 4504 a and 4504 b, which are provided over a first substrate 4501. In addition, a second substrate 4506 is provided over the pixel portion 4502, the signal line driver circuits 4503 a and 4503 b, and the scan line driver circuits 4504 a and 4504 b. In other words, the pixel portion 4502, the signal line driver circuits 4503 a and 4503 b, and the scan line driver circuits 4504 a and 4504 b are sealed together with a filler 4507, with the first substrate 4501, the sealant 4505, and the second substrate 4506. Packaging (sealing) is preferably performed, in such a manner, using a protective film (e.g., a bonding film or an ultraviolet curable resin film), a cover material, or the like with high air-tightness and little degasification.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503 b, and the scan line driver circuits 4504 a and 4504 b, which are formed over the first substrate 4501, each include a plurality of thin film transistors. FIG. 14B illustrates the thin film transistor 4510 included in the pixel portion 4502 and the thin film transistor 4509 included in the signal line driver circuit 4503 a.

As the thin film transistors 4509 and 4510, any of the thin film transistors described in the above embodiments can be employed.

A first electrode 4517 that is a pixel electrode of the light-emitting element 4511 is electrically connected to a source electrode or a drain electrode of the thin film transistor 4510. Note that the structure of the light-emitting element 4511 is not limited to the stacked structure in this embodiment, which includes the first electrode 4517, the light-emitting layer 4513, and a second electrode 4514. The structure of the light-emitting element 4511 can be changed as appropriate depending on the direction in which light is extracted from the light-emitting element 4511, or the like.

As for the first electrode 4517 and the second electrode 4514, an electrode that functions as a cathode can be formed using a conductive film that has a small work function and reflects light. For example, the electrode that functions as a cathode is preferably formed using a material such as Ca, Al, MgAg, or AlLi. An electrode that functions as an anode is formed using a light-transmitting conductive material. For example, a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added may be used.

A partition 4520 is formed using an organic resin film, an inorganic insulating film, organic polysiloxane, or the like. It is particularly preferable that the partition 4520 be formed using a photosensitive material to have an opening over the first electrode 4517 so that a sidewall of the opening is formed as an inclined surface with continuous curvature.

The light-emitting layer 4513 may be formed using a single layer or a plurality of layers stacked.

A protective film may be formed over the second electrode 4514 and the partition 4520 in order to prevent oxygen, hydrogen, water, carbon dioxide, or the like from entering the light-emitting element 4511. As the protective film, a silicon nitride film, a silicon nitride oxide film, a DLC film, or the like can be formed.

A variety of signals are supplied from FPCs 4518 a and 4518 b to the signal line driver circuits 4503 a and 4503 b, the scan line driver circuits 4504 a and 4504 b, the pixel portion 4502, or the like.

In this embodiment, an example is described in which a connection terminal electrode 4515 is formed using the same conductive film as the first electrode 4517 of the light-emitting element 4511, and a terminal electrode 4516 is formed using the same conductive film as the source or drain electrode of the thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to a terminal included in the FPC 4518 a via an anisotropic conductive film 4519.

The substrate located in the direction in which light is extracted from the light-emitting element 4511 needs to have a light-transmitting property. As a substrate having a light-transmitting property, a glass plate, a plastic plate, a polyester film, an acrylic film, and the like are given.

As the filler 4507, an ultraviolet curable resin, a thermosetting resin, or the like can be used, in addition to an inert gas such as nitrogen or argon. For example, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, a silicone resin, polyvinyl butyral (PVB), ethylene vinyl acetate (EVA), or the like can be used. In this embodiment, an example in which nitrogen is used for the filler is described.

If needed, an optical film, such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retardation plate (a quarter-wave plate or a half-wave plate), or a color filter, may be provided on a light-emitting surface of the light-emitting element. Furthermore, an antireflection treatment may be performed on a surface thereof. For example, anti-glare treatment by which reflected light can be diffused by projections and depressions on the surface so as to reduce the glare can be performed.

The signal line driver circuits 4503 a and 4503 b and the scan line driver circuits 4504 a and 4504 b may be formed using a single crystal semiconductor or a polycrystalline semiconductor over a substrate separately prepared. Alternatively, only the signal line driver circuits or part thereof or only the scan line driver circuits or part thereof may be separately formed and mounted. This embodiment is not limited to the structure illustrated in FIGS. 14A and 14B.

Through the above-described steps, a high-performance light-emitting display device (display panel) can be manufactured. This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

Embodiment 11

In this embodiment, examples of electronic devices each including the display device described in the above embodiment will be described.

FIG. 15A illustrates a portable game machine, which includes a housing 9630, a display portion 9631, a speaker 9633, operation keys 9635, a connection terminal 9636, a memory medium insert portion 9672, and the like. The portable game machine illustrated in FIG. 15A has a function of reading a program or data stored in the recording medium to display it on the display portion, a function of sharing information with another portable game machine by wireless communication, and the like. Note that the portable game machine illustrated in FIG. 15A can have a variety of functions without limitation to the above-described functions.

FIG. 15B illustrates a digital camera, which includes a housing 9630, a display portion 9631, a speaker 9633, operation keys 9635, a connection terminal 9636, a shutter button 9676, an image receiving portion 9677, and the like. The digital camera with a television reception function illustrated in FIG. 15B has a function of photographing a still image and/or a moving image, a function of automatically or manually correcting the photographed image, a function of obtaining various kinds of information from an antenna, a function of storing the photographed image or the information obtained from the antenna, a function of displaying the photographed image or the information obtained from the antenna on the display portion, and the like. Note that the digital camera with the television reception function illustrated in FIG. 15B can have a variety of functions without limitation to the above-described functions.

FIG. 15C illustrates a television set, which includes a housing 9630, a display portion 9631, speakers 9633, operation keys 9635, a connection terminal 9636, and the like. The television set illustrated in FIG. 15C has a function of converting an electric wave for television into an image signal, a function of converting the image signal into a signal suitable for display, a function of converting a frame frequency of the image signal, and the like. Note that the television set illustrated in FIG. 15C can have a variety of functions without limitation to the above-described functions.

FIG. 16A illustrates a computer, which includes a housing 9630, a display portion 9631, a speaker 9633, operation keys 9635, a connection terminal 9636, a pointing device 9681, an external connection port 9680, and the like. The computer illustrated in FIG. 16A has a function of displaying a various kinds of information (e.g., a still image, a moving image, and a text image) on the display portion, a function of controlling processing by various kinds of software (programs), a communication function such as wireless communication or wired communication, a function of being connected to a variety of computer networks with the communication function, a function of transmitting or receiving a variety of data with the communication function, and the like. Note that the computer illustrated in FIG. 16A can have a variety of functions without limitation to the above-described functions.

FIG. 16B illustrates a mobile phone, which includes a housing 9630, a display portion 9631, a speaker 9633, operation keys 9635, a microphone 9638, and the like. The mobile phone illustrated in FIG. 16B has a function of displaying various kinds of information (e.g., a still image, a moving image, and a text image) on the display portion, a function of displaying a calendar, a date, the time, or the like on the display portion, a function of operating or editing the information displayed on the display portion, a function of controlling processing by various kinds of software (programs), and the like. Note that the mobile phone illustrated in FIG. 16B can have a variety of functions without limitation to the above-described functions.

FIG. 16C illustrates an electronic paper (the device is also referred to as an e-book reader), which includes a housing 9630, a display portion 9631, operation keys 9635, and the like. The e-book reader illustrated in FIG. 16C has a function of displaying various kinds of information (e.g., a still image, a moving image, and a text image) on the display portion, a function of displaying a calendar, a date, the time, or the like on the display portion, a function of operating or editing the information displayed on the display portion, a function of controlling processing by various kinds of software (programs), and the like. Note that the e-book reader illustrated in FIG. 16C can have a variety of functions without limitation to the above-described functions.

In each of the electronic devices described in this embodiment, off-state current can be reduced in a plurality of pixels included in the display portion. Thus, an electronic device that includes a display device in which voltage can be held in a storage capacitor for a long time and which consumes less power at the time of displaying a still image or the like can be obtained. Further, by increasing aperture ratio, a high-definition display portion in display device can be achieved.

This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

This application is based on Japanese Patent Application serial no. 2009-251060 filed with Japan Patent Office on Oct. 30, 2009, the entire contents of which are hereby incorporated by reference. 

The invention claimed is:
 1. A semiconductor device comprising: an oxide semiconductor film; a gate insulating film over the oxide semiconductor film; and a first gate electrode and a second gate electrode, each of the first gate electrode and the second gate electrode facing a side surface of the oxide semiconductor film with the gate insulating film interposed therebetween, wherein the first gate electrode and the second gate electrode are electrically connected to each other, and wherein the oxide semiconductor film comprises indium and zinc.
 2. The semiconductor device according to claim 1, wherein the first gate electrode is positioned on a side opposite to the second gate electrode with the oxide semiconductor film interposed.
 3. The semiconductor device according to claim 1, wherein the oxide semiconductor film further comprises gallium.
 4. The semiconductor device according to claim 1, wherein the oxide semiconductor film is intrinsic or substantially intrinsic.
 5. The semiconductor device according to claim 1, wherein a carrier concentration of the oxide semiconductor film is lower than or equal to 5×10¹⁴ cm⁻³.
 6. The semiconductor device according to claim 1, wherein a concentration of hydrogen in the oxide semiconductor film is lower than or equal to 5×10¹⁹ cm⁻³.
 7. A semiconductor device comprising: an oxide semiconductor film; a gate insulating film over the oxide semiconductor film; and a first gate electrode and a second gate electrode, each of the first gate electrode and the second gate electrode facing a side surface of the oxide semiconductor film with the gate insulating film interposed therebetween, wherein the first gate electrode and the second gate electrode are electrically connected to each other, wherein the oxide semiconductor film comprises indium and zinc, and wherein a degree of crystallization in the oxide semiconductor film is 90% or more.
 8. The semiconductor device according to claim 7, wherein the first gate electrode is positioned on a side opposite to the second gate electrode with the oxide semiconductor film interposed.
 9. The semiconductor device according to claim 7, further comprising a source electrode and a drain electrode, wherein the oxide semiconductor film is positioned the source electrode, wherein the drain electrode is positioned over the oxide semiconductor film, and wherein the gate insulating film is positioned over the drain electrode.
 10. The semiconductor device according to claim 7, wherein the oxide semiconductor film further comprises gallium.
 11. The semiconductor device according to claim 7, wherein the oxide semiconductor film is intrinsic or substantially intrinsic.
 12. The semiconductor device according to claim 7, wherein a carrier concentration of the oxide semiconductor film is lower than or equal to 5×10¹⁴ cm⁻³.
 13. The semiconductor device according to claim 7, wherein a concentration of hydrogen in the oxide semiconductor film is lower than or equal to 5×10¹⁹ cm⁻³.
 14. A semiconductor device comprising: an insulating film over a substrate; a source electrode over the insulating film; an oxide semiconductor film over the source electrode; a drain electrode over the oxide semiconductor film; a gate insulating film over the drain electrode; and a first gate electrode and a second gate electrode, each of the first gate electrode and the second gate electrode facing a side surface of the oxide semiconductor film with the gate insulating film interposed therebetween, wherein the first gate electrode and the second gate electrode are electrically connected to each other, and wherein the oxide semiconductor film comprises indium and zinc.
 15. The semiconductor device according to claim 14, wherein the first gate electrode is positioned on a side opposite to the second gate electrode with the oxide semiconductor film interposed.
 16. The semiconductor device according to claim 14, wherein the oxide semiconductor film further comprises gallium.
 17. The semiconductor device according to claim 14, wherein the oxide semiconductor film is intrinsic or substantially intrinsic.
 18. The semiconductor device according to claim 14, wherein a carrier concentration of the oxide semiconductor film is lower than or equal to 5×10¹⁴ cm⁻³.
 19. The semiconductor device according to claim 14, wherein a concentration of hydrogen in the oxide semiconductor film is lower than or equal to 5×10¹⁹ cm⁻³. 